Vacuum fluorescent (hereinafter "VF") display arrangements are known. All present filament bias methods for large VF displays appear to use a center-tapped transformer as the bias source. The transformer is either run from the alternating current (hereinafter "AC") power mains or as part of a direct current (hereinafter "DC")-to-DC converter. Requiring the storage element in a DC-to-DC converter to have a floating center tapped output greatly increases the cost of the energy storage element, typically about $1.50. A DC-to-DC converter for the anodes and grids of a VF display only has to supply about 20 milli Ampere (hereinafter "mA") and the required inductor may cost less than $0.10.
A prior art DC arrangement 100 is shown in FIG. 1. There is shown a display arrangement comprising a first display digit 101, a second display digit 111, and a third display digit 121. The display digit 101 includes an anode 102, a grid 103, a cathode 104, and a grid terminal 107. The display digit 111 includes an anode 112, a grid 113, a cathode 114, and a grid terminal 117. The display digit 121 includes an anode 122, a grid 123, a cathode 124, and a grid terminal 127. Note the anodes 102, 112, and 122 are connected to +30 volts. Note the cathodes 104, 114 and 124 are connected in series. Thus, the potential at cathode 104 will be near +5 volts, the potential at cathode 114 will be near +2.5 volts, and the potential at cathode 124 will be near ground. Assuming that a voltage pulse of +30 volts is applied to each of the three individual grid terminals 107, 117, and 127, this would result in varying grid (103, 113, 123)-to-cathode (104, 114, 124) potentials, as shown in FIG. 2.
Referring now to FIG. 2, there are shown three (3) typical signal waveforms 201, 211, and 221 of pulses appearing at the grids 103, 113, and 123, respectively, of the arrangement 100 of FIG. 1. Each waveform (201, 211, 221) represents the voltage measured from the grid (103, 113, 123) with respect to the cathode (104, 114, 124) of the respective display device (101, 111, 121). Thus, waveform 201 represents the voltage at grid 103 (and terminal 107) measured with respect to cathode 104; waveform 211 represents the voltage at grid 113 (and terminal 117) measured with respect to cathode 114; and, waveform 221 represents the voltage at grid 123 (and terminal 127) measured with respect to cathode 124. The waveform pulses 201, 211, and 221 have associated magnitudes 202, 212, and 222, respectively. It will be recalled that, since cathodes 104, 114, and 124 are connected in series, then the voltage (with respect to ground) at the cathodes 104, 114, and 124 will vary. As a result, since the potential at the display device grids 103, 113, and 123 is a uniform +30 volts, then the respective grid-to-cathode voltages will also vary. Thus, since V.sub.104 &gt;V.sub.114 &gt;V.sub.124, this results in the relationship V.sub.202 &lt;V.sub.212 &lt;V.sub.222. The display intensity (or illuminating energy) of any device, of course, is directly related to the magnitude of the grid-to-cathode potential. Since the grid-to-cathode potentials of devices 103, 113, and 123 vary, then this, of course, results in the display luminescence energy in devices 103, 113, and 123 varying. This is because the varying potential causes electrons to hit the anode with varying speed causing a variation in display intensity.
As a result, there is a need for an improved display bias arrangement.